Well, there's no unified legal frameworks for them across all of _Europe_, for starters.

But in many places there's no need to get the members to authorize the strike, the union itself holds that power (I am at least 99% sure that's the case in Germany and France, at the very least.)

In Germany members have to vote for a strike and also for ending a strike. The term is "Urabstimmung".

Right, yes, I oversimplified.

You don’t need a vote for „Warnstreik” (warning strike), which frequently is enough to make a point, which makes the „real”/open-ended strikes much more rare than otherwise.

AFAICT all strikes I’ve ever seen happen here (the plethora of different train and airport unions) were technically warning strikes and not „real” ones; but they were enough to get the unions their wins.